Apparatus and methods for automatically inspecting photomasks of reticles used in manufacturing VLSI circuits have been described in the literature and have been commercially available for a number of years.
These systems generally employ an optical sensor to provide an electronic representation of the inspected object so that a comparison to another similar object or electronic representation can be made. In order to detect non-random, i.e. repeating, defects the comparison is generally made against a known good reference. If the reference is the design database from which the inspected article was manufactured, a generally complete set of defect types may be detected. An inspection system may be characterized by the different types of articles it can inspect, the type of reference it uses, the types and sizes of defects it detects, and the time it requires to inspect a complete article. Modern VLSI technology typically requires the detection of 0.5 micron defects on a single die 5.times. reticle in 10 minutes.
Inspection apparatus is disclosed in U.S. Pat. Nos. 4,247,203 and 4,347,001, both to Levy et al. The apparatus described in those patents locates faults in the photomask by simultaneously comparing adjacent dies on the photomask and locating differences. Because a known good die is not used in this type of inspection apparatus, only random, i.e. non-repeating, faults are generally identifiable, and not repeating faults.
U.S. Pat. No. 4,579,455, also to Levy et al, describes attempts to improve the detection efficiency of inspection apparatus especially at photomask pattern edges. An area subtraction technique is used to identify defects by detecting differences between substantially duplicate die patterns in a photomask. Two square window matrices of seven rows and seven columns of adjacent pixels are defined for corresponding areas of two die patterns in a single photomask. The center 3.times.3 matrix of each window matrix is defined as a comparison matrix with each window matrix having twenty five unique subsets of 3.times.3 adjacent pixels within its boundaries; one in the center and twenty four others that are offset by one or two pixels from the center in one or both directions.
An error value is then calculated for each subset of each matrix by summarizing the squares of the differences between each of the nine pixel values of each subset and the corresponding pixel value of the opposite comparison matrix. If there is no defect, and misalignment between the two representations is less than two pixels, at least one error value will be less than a threshold error value. If none of the twenty five error values relating to one comparison matrix are less than the threshold value, a defect is assumed to be located within the comparison matrix or within the opposite window matrix. The magnitude of the threshold error is then automatically varied according to the number of edges within the window matrices to compensate for edge induced errors.
U.S. Pat. No. 4,532,650 to Wihl et al describes attempts to improve the defect efficiency of inspection apparatus especially near photomask pattern corners. The detection process is based upon using a vector gradient within a matrix to develop candidate and canceller information for each representation. This information is then logically manipulated to qualify the data obtained and to determine whether or not a defect has been detected.
U.S. Pat. No. 4,805,123 to Specht et al describes a photomask and reticle inspection system in which a first stream of data representing the image content of a selected surface area of an object to be inspected is compared to a second stream of data representing the intended image content of the selected surface area of the object. After misalignment between stored portions of the two streams of data is detected and corrected, using shifts of an integral number of pixels and/or subpixel interpolation methods, subportions are compared in order to detect differences therebetween in excess of a predetermined threshold.
U.S. Pat. No. 4,926,489 to Danielson et al. describes a die-to-database reticle inspection system including timing control means which is responsive to misalignment between the inspected object and the database. The timing control means is used to generate control signals which adjust the relative output signal rates of the scanner and the database thereby maintaining the misalignment below a predetermined limit.